The SPF (Sun Protection Factor) is used as a scale representing the ultraviolet radiation protection effect of cosmetic products for preventing sunburn due to ultraviolet radiation (so-called sun protection products). This SPF, which is an index indicating the effect of skin protection from sunburn due to ultraviolet radiation and sunburn preventions is defined by the value obtained by dividing the amount of ultraviolet radiation necessary for causing slight redness in the case of using a sun protection product by the amount of ultraviolet radiation necessary for causing slight redness in the case of not using a sun protection product. This means that with, for example, a sun protection product of SPF 10, the same sunburn (erythema) as that on bare skin is caused by exposure to ten times as much ultraviolet radiation as in the case of causing sunburn on bare skin. In measuring the SPF, artificial light (a solar simulator) very close to sunlight is adopted in place of sunlight that may vary in value depending on the season or location. The measurement is performed by exposing each of unprotected skin and protected skin to a certain amount of ultraviolet radiation and determining the next day whether erythema has been caused.
Using the SPF measured based on the above-described method makes it possible to objectively evaluate the ultraviolet radiation protection effect of sun protection products. However, the above-described method necessitates the cooperation of a large number of volunteers of specific skin types, thus requiring a large amount of money and a large number of days. Accordingly, it has been desired to develop an in vitro and simple method of calculating in vitro predicted SPFs having high correlation with the in vivo SPFs obtained by the above-described method for, for example, evaluation of the ultraviolet radiation protection effect of a product under development.
Conventionally known methods of evaluating an ultraviolet radiation protection effect by in vitro measurement includes a dilution method that measures the ultraviolet radiation absorbance or transmittance of a sample diluted with an organic solvent in a quartz cell and a thin film method that measures the ultraviolet radiation absorbance or transmittance of a sample formed into a film having even thickness on a quartz plate. These conventional methods are significant in understanding characteristics such as the absorption maximum wavelength and protection wavelength region of an ultraviolet absorber, but cannot predict the SPF. This is because these methods for evaluating an ultraviolet radiation protection effect differ greatly from methods for measuring in vivo SPFs. Further, the biological reaction represented by the SF depends on the ultraviolet wavelength, and the erythema reaction is likely to occur at some ultraviolet wavelengths and is less likely to occur at other ultraviolet wavelengths. Therefore, it has been considered necessary to take the effect on a living body into consideration on a wavelength basis.
With respect to the above-described two problems, Non-Patent Document 1 applies a sample on a medical tape serving as a skin substitute film and measures the spectral transmission spectrum of the sample. This measurement result is subjected to calculation by the Diffey & Robson formula to calculate the SPF. This Diffey & Robson formula succeeds in solving the above-described problems by countering the wavelength dependence of the erythema reaction as a human biological reaction by using an erythema factor disclosed in Patent Document 2.
However, in vivo SPFs include various factors such as an individual difference, a regional difference, an age difference, a gender difference, and a skin type difference. Therefore, it has been considered a problem that it is actually very difficult to predict the SPF with accuracy based only on the single example of the erythema factor.
Therefore, an evaluation method has been proposed that can predict the in vitro SPFs of even unknown samples not by adopting only the erythema factor but by deriving an arithmetic expression capable of obtaining statistically high correlation from the relationship between a large number of samples with known in vivo SPFs and their spectral transmission spectra. (See, for example, Patent Document 1.) This evaluation method makes it possible to obtain in vitro predicted SPFs with accuracy and has eliminated variation factors arising from an individual difference, a regional difference, an age difference, a gender difference, a skin type difference, etc.
[Non-Patent Document 1] Journal of the Society of Cosmetic Chemists (1989) 40:33, 127-133
[Non-Patent Document 2] CIE Journal (1987) 6:1, 17-22
[Patent Document 1] U.S. Pat. No. 3,337,832